For a variety of commercially significant purposes it is desirable to perform nondestructive tests to measure the electrical conductance, conductivity, or resistivity of a sample.
For example, during semiconductor product manufacturing, there is a need to measure the conductivity of various conductive thin films on semiconductor wafers and integrated circuits in a nondestructive manner. Also during semiconductor product manufacturing, it is useful to perform stress measurements on semiconductor wafers and integrated circuits in a nondestructive manner.
It is well known that such measurements can be obtained by eddy current testing. One conventional apparatus for performing eddy current testing on a sample is described in U.S. Pat. No. 4,000,458, issued Dec. 28, 1976. Another is described in Jeanneret, et al., "Inductive Conductance Measurements in Two-Dimensional Superconducting Systems," Applied Phys. Lett. 55 (22), pp. 2336-2338 (Nov. 27, 1989). The Jeanneret, et al., apparatus employs two coils, both positioned above the sample: a drive coil (of radius 2.05 mm), and an astatically wound receiver coil (having radius 1.2 mm) coaxially mounted within the drive coil. The receiver coil has a first section (wound with right-handed helical geometry) and a second section (wound with left-handed helical geometry). The lower end of the drive coil is positioned at a first known distance (0.3 mm) above the sample, and the lower end of the lower section of the receiver coil is positioned at a second known distance above the sample, where the second distance is much (e.g., an order of magnitude) smaller than the first distance. As the drive coil is driven by an AC voltage source (at a frequency of 70 kHz), the in-phase and quadrature components of the voltage at the receiver coil are measured by "conventional lock-in techniques or by an ac mutual-inductance bridge." The resulting voltage data can be processed (with data indicating the coils' distance from the sample) to determine the sample's complex conductance.
Several conventional techniques for processing in-phase and quadrature voltage data obtained during eddy current testing are described in the Nondestructive Testing Handbook, Second Edition, edited by R. C. McMaster, American Society for Nondestructive Testing, Inc. (1986), Volume 4-Electromagnetic Testing, at pages 218-222. However, such conventional techniques require accurate knowledge of the separation between the sample and an eddy current probe (comprising drive coil and sense coil) at one or more positions of the probe relative to the sample.
In typical applications of eddy current testing, accurate measurement (or prior knowledge) of the separation between the eddy current probe and the sample requires complicated and expensive equipment. For example, U.S. Pat. No. 4,302,721 describes an eddy current testing apparatus which employs complicated acoustic wave measurement equipment to measure eddy current probe-to-sample separation.
Other typical applications of eddy current testing require maintenance of the eddy current probe at a constant, precisely repeatable, distance from the sample. For example, U.S. Pat. No. 4,849,694, issued Jul. 18, 1989, assumes that a sample's resistivity or thickness is known, and employs eddy current measurements to determine the other of the sample's resistivity or thickness in a manner requiring that the measurement apparatus maintain the eddy current probe at a constant, precisely repeatable distance "d" from the sample surface. This reference teaches that a precision optical microscope is preferably used to precisely position the eddy current probe at the distance "d" from the sample.
Until the present invention, it was not known how to perform eddy current testing to obtain accurate conductance or conductivity measurements on a sample without the need to measure the separation between the probe and the sample or to perform measurements at a precisely maintained probe-to-sample separation. The eddy current testing method of the present invention provides a convenient and inexpensive way to obtain conductance and/or conductivity measurements on a sample without the need to measure separation between an eddy current probe and a sample.